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Why Wireless?
Wireless Spectrum:
< 1 GHz: Long range, good penetration (700/850/900 MHz for LTE)
1-6 GHz: Balance (2.4/5 GHz WiFi, 2100/2600 MHz LTE)
6-30 GHz: Mid-band 5G (3.5 GHz, 28 GHz)
30-300 GHz: mmWave 5G (38/60/77 GHz)
Free Space Path Loss (Friis):
PL(dB) = 20log(4πd/λ) = 20log(4πdf/c)
At 2.4GHz, d=100m: PL ≈ 80 dB
Log-distance model:
PL(d) = PL(d₀) + 10n·log(d/d₀) + Xσ
n = path loss exponent (2 free space, 3-4 urban, 4-6 indoor)
Xσ = shadowing (log-normal, σ=6-10dB typical)
Multipath: Signal arrives via multiple paths
Delay spread (στ): RMS spread of arrival times
→ Coherence bandwidth Bc = 1/(50στ)
Doppler spread (BD): due to mobility
Doppler frequency fd = v·cos(θ)/λ
→ Coherence time Tc = 0.423/fD
Classification:
Frequency selective vs flat fading: compare symbol period to στ
Fast vs slow fading: compare symbol period to Tc
Rayleigh fading: no dominant LOS component
Envelope: p(r) = (r/σ²)exp(-r²/2σ²)
Worst case for wireless links
Rician fading: dominant LOS + multipath
K-factor = LOS power / scattered power
K→∞: AWGN, K→0: Rayleigh
Goal: send redundant copies via independent paths
→ reduce probability all copies are in deep fade simultaneously
Types:
1. Space diversity: multiple antennas (spacing > λ/2)
2. Frequency diversity: same signal on different frequencies
3. Time diversity: repeat transmission (interleaving + coding)
4. Polarization diversity: vertical + horizontal antennas
Combining techniques:
Selection Combining (SC): choose best SNR branch
Equal Gain Combining (EGC): co-phase and sum all
Maximal Ratio Combining (MRC): weighted sum → optimal
With L branches (independent fading):
Outage probability ≈ (γth/γ̄)^L (much better than single antenna!)
Spatial Multiplexing:
Nt transmit, Nr receive antennas
Channel matrix H: Nr × Nt
y = Hx + n
SVD: H = UΣV^H
min(Nt,Nr) parallel spatial streams
Shannon capacity:
C = Σᵢ log₂(1 + σᵢ²·P/(Nt·N₀)) bits/s/Hz
(sum over min(Nt,Nr) singular values)
At high SNR: C ≈ min(Nt,Nr) × log₂(SNR)
MIMO multiplies capacity by min(Nt,Nr)!
Beamforming:
Steer antenna beam toward desired user
Reject interference from other directions
Phased array: adjust amplitude+phase of each antenna element
Analog beamforming: single RF chain, one beam at a time
Digital beamforming: one RF chain per antenna, flexible
Hybrid: fewer RF chains than antennas (5G compromise)
Massive MIMO (5G): 64-256 antennas at base station
→ High beamforming gain, multi-user MIMO
→ Channel hardening: channel becomes more predictable
Network Elements:
UE (User Equipment) — phone, tablet, IoT device
gNB (5G base station) / eNB (4G)
└── Multiple sectors (3 typical, 120° each)
RAN (Radio Access Network)
Core Network: AMF, SMF, UPF (5G), EPC (4G)
Internet/IMS
Frequency Reuse:
Traditional: N=7 cluster (1/7 frequency reuse)
Modern: Frequency reuse 1 with interference coordination
ICIC (Inter-Cell Interference Coordination) in LTE
Dynamic TDD in 5G
Cell types:
Macro cell: high tower, 1-35km radius
Micro cell: 200m-2km, urban dense areas
Pico cell: 100m, hotspots
Femto cell: 10-50m, home/office (self-install)
Air Interface: LTE-FDD or LTE-TDD
Multiple access: OFDMA (DL), SC-FDMA (UL)
Bandwidth: 1.4, 3, 5, 10, 15, 20 MHz
Subcarrier spacing: 15 kHz
Modulation: QPSK, 16-QAM, 64-QAM, 256-QAM
MIMO: 2x2, 4x4, 8x8 (standard)
Peak: 150 Mbps DL (Cat-4), 1 Gbps (Cat-16)
Latency: ~30ms
LTE Channels:
Physical: PDSCH (data DL), PUSCH (data UL)
PDCCH (control), PBCH (broadcast)
Logical: BCCH, CCCH, DTCH, DCCH
Procedures:
Initial attach → Authentication → PDN connection → IP assigned
Idle mode: DRX (Discontinuous Reception) saves battery
Connected mode: HARQ retransmissions, link adaptation
Frequency ranges:
FR1: 450MHz - 6GHz (sub-6GHz)
FR2: 24.25GHz - 52.6GHz (mmWave)
Numerology (μ): scalable subcarrier spacing
μ=0: 15kHz (LTE compat), μ=1: 30kHz, μ=2: 60kHz
μ=3: 120kHz (mmWave), μ=4: 240kHz
Key features vs LTE:
✓ Massive MIMO (up to 256 antennas)
✓ Flexible numerology
✓ Network slicing
✓ MEC (Multi-access Edge Computing)
✓ NR-U (unlicensed spectrum)
✓ Dynamic TDD (slot format flexible)
Use cases:
eMBB (enhanced Mobile Broadband): fast data, video
URLLC (Ultra Reliable Low Latency): <1ms, 99.9999%
mMTC (massive Machine Type Comm): 1M devices/km²
DS-SS (Direct Sequence):
Data XOR with high-chip-rate PN code
Bandwidth spread by processing gain Gp = W/B
CDMA 3G uses DS-SS
Jam resistance, LPI, multiple access
FH-SS (Frequency Hopping):
Frequency changes according to PN sequence
Bluetooth: 79 channels, 1600 hops/sec
Military comms, GPS
Processing Gain:
Gp = W/B = Rc/Rb (chip rate / bit rate)
SNR_out = SNR_in × Gp
High Gp → more jam resistance
Q: Why does higher frequency attenuate more? A: Free space PL ∝ f² — higher frequency, smaller wavelength, smaller effective aperture of receive antenna → more path loss. Also: rain attenuation significant >10GHz, oxygen absorption at 60GHz.
Q: Channel estimation kaise hoti hai LTE mein? A: Pilot symbols (Reference Signals) known symbols at known positions in time-frequency grid. Receiver compare received vs known → estimate channel at pilots → interpolate to get H(f,t) → equalize data symbols.
Q: Full duplex vs half duplex — 5G mein kya approach hai? A: FDD (Frequency Division Duplex): UL/DL different frequencies — simultaneous. TDD (Time Division Duplex): same frequency, different time slots. 5G NR supports both. Dynamic TDD allows flexible UL/DL ratio per slot.
Complete Wireless Communications notes for B.Tech ECE Sem 7 — Channel models, Fading, MIMO, Cellular systems, 4G LTE, 5G NR, Satellite, spread spectrum with exam questions.
46 pages · 2.3 MB · Updated 2026-03-11
Signal multiple paths se receiver tak pahunchta hai — reflection, diffraction, scattering. Paths alag lengths → phase differences → constructive/destructive interference → signal level fluctuates. Fast fading (mobile speed) aur slow fading (shadowing).
Multiple Input Multiple Output — multiple transmit + receive antennas. Spatial multiplexing: multiple data streams simultaneously (capacity badhti hai). Diversity: reliability improve. Beamforming: directional gain. 4G/5G/WiFi mein essential.
Same frequency band doosre geographic cell mein reuse karna. Cluster size N = i²+ij+j². N=7 traditional, N=1 modern (interference management se). Reuse badhne se capacity badhti hai.
mmWave (24-100GHz): huge bandwidth available → Gbps speeds. Lekin: short range (100-300m), rain/foliage attenuation, no penetration through walls. Dense small cell deployment needed. High-speed indoor, stadiums, dense urban.
UE (phone) continuously measures signal strength of serving + neighbor cells. RSRP/RSRQ threshold cross → handover trigger → RRC reconfiguration → connect to new cell. Hard handover (LTE) vs soft handover (CDMA).
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